The present invention concerns a contactless smart card, of the RF (radio-frequency) type.
Numerous embodiments of this type of card are available on the market. They generally include an electronic module including a sensor coil associated with an integrated circuit, this coil allowing, on the one hand, RF signals to be transmitted, and on the other hand, the power necessary for the electronic module to operate to be supplied. U.S. Pat. No. 4,115,091 and European Patent No. 762 535 disclose examples of such embodiments. The simplest cards comprise only a code which can be read remotely, but more and more read/write smart cards can also be found including an EEPROM type memory capable of saving its data even in the absence of a power source. Writing in such memories requires a relatively large amount of power which has to be taken across the terminals of the sensor coil. This may pose problems, particularly in  less than  less than  hands free  greater than  greater than  access systems which constitute one of the major applications of such cards.
One solution for obtaining the power necessary for the card to operate at the greatest possible distance is to use a coil with a larger surface area allowing a maximum of flux to be picked up. Thus, round coils often have a diameter close to the width of the card and, in certain cases, rectangular coils are even used which cover practically the entire surface of the card. Another possibility consists in integrating antennas made of materials with high permeability in the card in order to concentrate the flux passing in proximity to the card at the centre of the sensor coil. The document WO. 98/52141 discloses such a system which allows coils of small diameter to achieve performances equivalent to those obtained with cards of large diameter.
However, a major problem remains. Indeed, in both the aforementioned cases, the maximum flux in the coil is only obtained when the card has a well defined orientation relative to the lines of flux generated by the reader. When the position of the card is changed, the flux quickly decreases and there are even numerous neutral positions where the flux in the coil is quite simply zero or, at least, close to zero. This obviously poses serious problems, particularly within the scope of  less than  less than  hands free  greater than  applications since the position of the card relative to the lines of flux generated by the reader may be very variable depending upon the person carrying it and the place where he puts his card.
The object of the present invention is precisely to provide a sensor coil/antenna combination allowing the aforementioned problem to be largely resolved by making the quantity of flux picked up by the coil much more independent of the relative position between the card and the lines of flux generated by the reader.
More precisely, the invention concerns a contactless RF-type smart card, including a sensor coil, an electronic circuit connected to the coil and elements made of material with high magnetic permeability acting as antenna, including a first part located substantially in the plane of the upper coil surface and a second part located substantially in the plane of the lower coil surface. It is characterised in that the antenna includes a connecting part between the first and second part, positioned at the centre of the coil, the latter being dimensioned relative to the antenna such that the voltages induced across its terminals are of the same order of magnitude, whether the antenna is parallel or perpendicular to the lines of magnetic flux generated by a reader device.